1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to an improved cascaded Raman fiber laser system based on a filter fiber.
2. Background Art
Cascaded Raman fiber lasers (CRFLs) are useful devices for generating laser outputs at wavelengths at which rare-earth ionic gain is not available. A CRFL provides a stepwise transition from a starting wavelength to a selected target wavelength. The stepwise transition is created through cascaded lasing of one or multiple Raman orders in a suitable Raman gain medium. A nested series of Raman cavities are created in the gain medium by, for example, a corresponding nested series of in-line reflective grating pairs. Each successive cavity in the series is separated in wavelength from the preceding cavity by the respective Raman Stokes shift introduced by Raman scattering in the preceding cavity. A CRFL is typically pumped using a high-power, continuous-wave (CW) laser, such as a cladding-pumped Yb-doped fiber laser.
FIG. 1 is a diagram of a current CRFL system configuration 20, in which a system output of 41 W of power at 1480 nm was demonstrated. The single-mode 1480 nm system output 70 is suitable for use as a high-power pump for core pumping of an erbium-doped fiber laser (EDFL) or an erbium-doped fiber amplifier (EDFA). As shown in FIG. 1, system 20 comprises two stages: a monolithic Yb-doped fiber laser 40 and a cascaded Raman resonator (CRR) 60.
In laser 40, the active gain medium is provided by a length of a double-clad Yb-doped fiber 42 operating in the region of 1000 nm to 1200 nm. A high reflector grating HR1 is provided at the input end 44 of fiber 42, and an output coupler grating OC1 is provided at the output end 46 of fiber 42. High reflector HR1, output coupler OC1, and fiber 42 function as a laser cavity 48. Pump power is provided to fiber 42 by a plurality of pumps 50, e.g., multimode 915 nm or 975 nm diode lasers, which are coupled to fiber 42 by means of a tapered fiber bundle TFB1. In the present example, the laser output 52 is single-mode radiation at a wavelength of 1117 nm.
The laser output 52 is used to launch a pump power input into the cascaded Raman resonator 60. Resonator 60 comprises a Raman-active fiber 62, having a small effective area and normal dispersion. The normal dispersion prevents modulation instability that would lead to supercontinuum generation at high powers. The small effective area leads to high Raman gain, and consequently multiple Stokes orders can be generated.
A first plurality of high reflector gratings HR2-HR6 are provided at the Raman fiber's input end 64, and a second plurality of high reflector gratings HR7-HR11 and an output coupler OC2 are provided at the Raman fiber's output end 66. An additional pump reflector recycles unused Yb radiation for increased efficiency. Input gratings HR2-HR6, output gratings HR7-HR11 and OC2, and Raman fiber 64 provide a nested series of Raman cavities 68. The respective wavelengths of each of the nested Raman cavities are configured to create a cascaded series of Stokes shifts over a broad range, increasing the wavelength of the 1117 nm laser output to a target wavelength of 1480 nm in a series of steps. Output coupler OC2 provides a system output 70 at a target wavelength of 1480 nm, which can then be used to pump an EDFA or EDFL in the fundamental mode.
The prior art system 20 suffers from a number of known drawbacks and limitations.
First, in increasing the output power to 41 W at 1480 nm, it was found that it was necessary to restrict the length of the Raman fiber 62 in resonator 60 in order to avoid unwanted Raman scattering to the next Stokes order, i.e., at 1590 nm.
Further, multiple reflectors at various wavelengths and positions in the system 20 combine to create coupled cavities. It will be seen that there are three reflectors at the laser wavelength of 1117 nm, i.e., gratings HR1, OC1 and HR7. In general this does not pose a problem for systems operating at relatively low power (e.g., 5 W output at 1480 nm). Recently, however, investigations have been undertaken with respect to power scaling of Raman fiber lasers. As mentioned above power levels as high as 41 W have been demonstrated from a CRR.
While high power has been demonstrated from such a system, the coupled cavity nature of the setup in FIG. 1 has serious implications on long-term reliable operation. In particular, the coupled cavity can cause the system to become unstable and generate pulses with sufficiently high peak power to damage components. The laser high reflector HR1 in particular has been found to be a weak link in the system, presumably due to the high power that propagates through it, and has been observed to fail under various conditions including, for example, using the system 20 to pump an erbium-doped fiber laser or amplifier. In addition, it is possible for light from intermediate Stokes orders generated in the Raman laser to propagate back into the Yb amplifier and back to the pump diodes, causing them to fail. Furthermore, light at the first Stokes shift is still within the gain bandwidth of Yb and is amplified before hitting the diodes. It will be apparent that this is also detrimental.